Long-Term Changes in Terrestrial Vegetation Linked to Shifts in a Colonial Seabird Population

Seabirds that form large colonies often act as biovectors that transport and concentrate large amounts of nutrients, metals, and contaminants from marine feeding areas to inland breeding grounds. This enrichment can potentially transform and structure primary productivity, vegetation communities, and species richness. In a previous paleolimnological study, we examined approximately 1700 years of population change in the world's largest colony of Leach's Storm-petrel (Hydrobates leucorhous) on Baccalieu Island (Newfoundland and Labrador, Canada) and, using a variety of proxies, we identified two peaks in colony around 500 and 1980 CE. Here, we analyzed the same sediment cores for fossil pollen assemblages to explore the effects of changing seabird populations on terrestrial vegetation. Aerial imagery revealed the island's vegetation cover that increased from about 23% to about 58% between 1940 and 2017, in part coinciding with the rapid colony growth until around 1980. Palynological analyses indicated shifts from tree and shrub habitat to storm-petrels' preferred habitat of fern, grass, and moss during peak seabird abundances around 500 and 1980 CE. Also, during peaks in colony size, nitrogen-fixing alder (Alnus spp.) decreased in relative abundance likely due to poorer competitive potential because of guano-derived nitrogen fertilization. Furthermore, we observed increases in fungal hyphae concurrent with the inferred size of the storm-petrel colony, providing the potential for a novel proxy to track burrowing seabirds in sediment records. Collectively, our data show that storm-petrels acted as ecosystem engineers by markedly modifying the island's vegetation cover and composition. If global seabird colonies continue to decline at current rates, there may be considerable bottom-up ramifications to terrestrial island ecosystems.